The invention consists of design improvements in the construction of electrolytic cells for electrowinning and electrorefining processes of nonferrous metals, with a novel mold and molding method and new formulations for three-layered polymer composite materials for the monolithic formation of the structural core with surface sealing coatings in the receptacles or containers of such cells.
There are currently several known designs for cell-type receptacles or containers intended for electrolytic refining and winning used in the purification and recovery of nonferrous metals. In order to obtain high purity cathodic copper, there are currently 2 well-established industrial electrolytic processes: electrorefining of melted copper anodes dissolved in sulfuric acid electrolytes, and electrowinning cathodic copper directly from copper sulfate electrolytes previously recovered by hydrometallurgic processes by extraction using lixiviated copper solvents in batteries. The receptacles for electrolytic cells used in both processes are similar, having a parallelepipedic geometry, being self-supporting, with suitable dimensions to lodge electrodes in the form of vertically positioned parallel laminar plates supported at each end at the upper edges of the side walls of the receptacle, and provided with means for electrolyte infeed and overflow. The design of the electrolytic cell receptacle itself is functional in order to accommodate the specific requirements of the corresponding electrolytic process. Currently, electrorefining cells typically operate with moderate electrolyte flows, at temperatures between 55xc2x0 C. and 75xc2x0 C., and the length/width ratio of the receptacle, in terms of the number of electrodes required for each cell, is generally less than 4; electrowinning cells, on the other hand, operate with much higher electrolyte flows, at lower temperatures, between 45xc2x0 C. and 55xc2x0 C., and their length/width ratio is typically greater than 4. Recent technological efforts to improve productivity of both electrolytic processes have shown tendencies toward greater current densities per electrode, higher electrolytic temperatures, and a higher number of electrodes per cell, i.e., with a length/width ratio that is typically 5 or 6.
One of the receptacles for electrolytic cells of the current state of the art is discussed in (Chilean) Patent No. 38,151, which characterizes a corrosive electrolyte receptacle or container used in electrolytic processes, where said receptacle consists of a polymer concrete box with side walls, a pair of opposite end walls, and a bottom, and each of said end walls has an inner and outer surface where a formation has been molded onto the outer surface of the end wall that extends from its upper and lower ends and that is intermediate between the sides of the wall; a depression has been formed on the upper end of the formation, which opens toward the inner surface of said end wall; and below the upper edge of the wall a generally vertical first discharge passage has been formed at a certain distance from the outer surface of the formation on the outer surface of the end wall; the discharge passage has a first opening on the end of the formation and a second opening adjacent to the lower end of the formation in order to drain off the electrolytes from the upper part of the receptacle, characterized in that it has a second passage formed in the end wall and running through the lower part of the wall to drain off the electrolytes from the lower part of the receptacle, wherein electrolytes may be removed from both the upper and lower part of the receptacle.
It also describes a formation with a second passage on the inner surface of the other end wall and forming part of the wall, said second passage running from the upper end of said wall downward to a position adjacent to the lower end, with a channel formed in the end wall and in the inner surface, with a covering over the channel that is open at its upper and lower ends, all for the purpose of distributing the electrolytes entering the cell.
In addition, a corrosion-resistant layer has been applied, which includes a surface layer of a material taken from a group that consists of vinyl ester resin and polyester resin, and a lining layer that consists of an inorganic fiber saturated with a material from a group that includes vinyl ester resin and polyester resin.
Said lining layer is made of about 20-30 wt % fiber and about 70-80 wt % resin. The inorganic fiber is fiberglass in the form of a sheet or layer, said sheet being made up of threads that are 12.7-50.8 mm long. The surface layer has a thickness of about 0.0254-0.0508 mm.
The polymer concrete consists of 10-19 wt % resin from a group that includes thermosetting vinyl ester and polyester resin. The modified resin includes 80-90% resin taken from a group consisting of vinyl ester and polyester resin, and the balance is a thinning agent, inhibitors, promoters, and a catalyst.
Finally, it describes a method that includes the phases of applying to the surface of a mold a surface layer made of a material taken from a group consisting of vinyl ester resins and polyester resins; applied to said surface layer is a lining layer consisting of a sheet of inorganic fiber saturated with a material taken from a group consisting of vinyl ester resins and polyester resins; a thermosetting resin from a group consisting of polyester resin and vinyl ester resin and an aggregate are mixed together, the mixture being continuously emptied into an inverted mold in which said surface layer and lining define the bottom, end, and side walls, thereby permitting said molded mixture to set, wherein the surfaces of the receptacle shall come into contact with the surfaces of the mold, which casts the smooth inner surfaces. Said layer is formed of threads that are 12.7-50.8 mm long and 0.0254-0.0508 mm thick. Said lining layer has about 20-30 wt % of fiber and about 70-80 wt % of resin. The aggregate includes a mixture that is 80-90 wt % of particles that are 6.35-0.79 mm in size; 10-15 wt % of particles taken from a group that consists of fine silica sand and fine silica powder and 0.9-5 wt % of particles from the group that consists of mica flakes whose approximate size is {fraction (1/64)} mm and of cut fiberglass threads 6.35-3.175 mm in length. In addition, the modified resin includes 80-90% resin from the group that consists of vinyl ester resin and polyester resin, and the balance is a thinning agent, inhibitors, promoters, and a catalyst.
Another (Chilean) patent, No. 35,466, refers to a compound material for use in molding containers or structures exposed to corrosive chemicals, particularly to corrosive acids, characterized in that it contains a plastic synthetic resin with an inert particulate filler composed of no less than 70 wt % of round particles whose diameter is on the order of less than 0.5 mm, with a total weight ratio of the particulate resin to the surrounding resin of 8:1 (that is, 11.1% resin content).
In the subordinate claims, the particulate material filler is described, which includes a fraction of about 40 wt % of the total filler of particles whose size ranges from 0.5-1 mm, and a fraction of about 15 wt % of the total filler of particles whose size varies between 1-1.75 mm and 1.75-3 mm.
Another receptacle for electrolytic processes of winning or refining nonferrous metals uses the concept of an inner container made of a two-layered polymer composite material, with the body of said container being preformed on an inverted mold by several successive applications of a first polymer composite material consisting of a base of fiberglass layers saturated with high corrosion-resistant polyester/vinyl ester resin contents. As the layers of polymer composite material closest to the surface of the mold cure, the thickness of the walls and bottom of the inner container imparts sufficient structural strength so that it may itself form the core mold for the electrolytic receptacle, which is then formed in a second phase of the manufacturing process. At the desired distance from the perimeter of the inverted inner container (acting as core mold), vertical molds are installed to vertically form the side and end walls and the thickness of the bottom of the electrolytic receptacle. The volume of the cavities defined by the molds so assembled is filled all around the inner container with a second polymer composite material based on a mixture of polyester/vinyl ester resin reinforced with particulate aggregate. The assembled receptacle is mechanically vibrated to compact the polymer concrete around the inner preformed container of fiberglass-reinforced polymer composite material. When the mass of the surrounding second polymer composite material cures, it does so joined to the outer layer of the first fiberglass-reinforced plastic material of the inner container/mold, thereby producing a chemical bond between the two polymer composite materials.
Although electrolytic cells constructed of polymer materials of the state of the art provide such advantages as improved ease of operation, productivity, and lower costs when compared to the cement concrete cells with corrosion resistant coatings of lead or plastic that they replaced, they still present significant disadvantages and technical shortcomings. The electrolytic cells of polymer concrete constructed according to the technology and the patents cited have experienced massive failures in various copper electrorefining and electrowinning plants in Chile, North America, and Europe. Defects persist in regard to both the absolute impermeability required of the cells while in operation, and significant variability in tolerances as to dimensions, structural strength, durability over time, as well as high manufacturing costs. The high costs result from the use of expensive polymer compound materials together with frequent and costly factory finishes, as well as a higher volume of polymer concrete material applied in the construction of the receptacle than is strictly necessary, which makes them heavier than the receptacles for cells of the proposed design according to the invention; as well as defective or non-existent chemical barriers or surface seals, and poorly specified structural reinforcement on the polymer concrete of the receptacles, which significantly affect their impermeability, safety, and durability and makes them difficult to clean, maintain, and above all to successfully repair cracks, so as to be able to recover their impermeability reliably.
The most important defects that cause premature breakdown and, in general, low reliability in the performance of the current polymer concrete electrolytic cells may be traced to such defects as:
Non homogeneousness and inconsistencies in the structural polymer concrete. These defects may be directly attributed to insufficient specifications and lack of rigorous control over raw materials, to deficient formulations for the polymer composite materials with excess resin, to mixing processes that are not homogeneous, and curing that lacks uniformity or is defective in regard to excessive solidification contraction, porosity due to improper compacting of the mixture in the mold, cracks due to irregular contraction of the polymer composite materials, cracks caused by defective molds, etc.
Added to the above-mentioned defects in the material and in forming and molding processes are ineffective mold designs that consistently produce cell receptacles that present variable nominal measurements and often random deformed geometry as well, which makes it more difficult, costly, and time-consuming to install and level them on site; the current state of the art views molds as devices that merely impart shape and not as true chemical reactors, whose characteristics affect the curing, properties, and condition of the composite polymer material; as a consequence of the above, the internal stresses in the material of finished cells according to the current state of the art are unacceptably high, particularly because the finished cells are not post-cured, which leaves them more susceptible or disposed to early breakdown due to cracks developed in the material during handling, shipping, and installation of cell receptacles made of a characteristically fragile material.
To the foregoing, we can add cell receptacle designs that are characterized by a parallelepipedic geometry with excessively thick walls and bottoms, particularly on the front and bottom walls as compared to the side walls, formed on the basis of materials with high resin content, and above all with the forms of the receptacle walls and bottom characterized by horizontal and vertical vertices with acute edges. The distribution of the volume of the material in conventional parallelepipedic geometry with acute vertices is not optimal for resisting the stresses to which cells are subjected, particularly thermal stresses caused by the contraction/expansion of the polymer concrete resulting from thermal gradients or differences between the temperatures of the inner surfaces in contact with hot electrolytes and the outer surfaces exposed to the outside environment or to contiguous cells. These thermal gradients, or, their sudden changes, may often cause cracks or fissures in the polymer concrete of the stressed bottom or walls which travel through current inner coatings and seals, resulting in leaks of corrosive electrolytes; and defects in regard to the cells being securely supported by and attached to their foundations, in order to ensure good seismic resistance and to protect the integrity of the cells during significant seismic events.
Finally, the internal reinforcement of the polymer concrete structure is under-specified with categories of materials that are not sufficiently corrosion resistant to sulfuric electrolytes, and are also defectively designed and installed merely to provide nominal protection to prevent disintegration of the cell material in the event of seismic catastrophes (catastrophes that, fortunately, have not yet occurred), and not for their primary function (in the event fissures in the material were to develop), which is to keep to a minimum the spreading of any fissures encountered in the material, so as to permit recovery of the structural integrity and impermeability of the cells by injecting liquid resin in the cracks. As the injected resin cures, it contracts and closes the fissure, adhering the material and sealing any leaks from the cells, thereby ensuring their impermeability; the reinforcement material is often based on fiberglass, which has very low resistance to acid corrosion by sulfuric electrolytes (Class E), and this fiberglass is also improperly dosed or poorly applied, which contributes to the formation of fissures and the loss of impermeability of the cells in the medium term.
None of the above-mentioned problems or disadvantages are fully or coherently resolved by the current state of the art.
The advantages of the improved electrolytic cells according to the invention are as follows:
With the feedback of results and problems encountered in the past 10 years concerning some 14,000 polymer concrete cells in plants for the electrorefining and electrowinning of copper, it has been possible to determine that the greatest structural stress to which cells are subjected during operation is thermal in origin and is generated by the effect of the difference between the temperature of the electrolytes inside the cell and the temperature of its external surroundings, creating temperature gradients on the inner and outer surfaces of the walls and the bottom of the cell. The concentrations of typical tensile stresses in specific areas of the electrorefining cell are, for example, more severe (indicated by structural analysis using the finite element method and taking into consideration relatively higher operating temperaturesxe2x80x94typically 58-75xc2x0 C.), and are generated by these thermal gradients between the temperatures on different areas of the inner surfaces and between them and the outer surfaces of the structural core of polymer concrete material of the walls and bottoms of the cells. In the invention, these are significantly reduced or eliminated by three strategies applied individually or jointly:
A) Introducing in the design of the receptacle wide radii of curvature in all intersections or vertices of the walls and between the walls and the bottom.
B) Introducing in the manufacture of the receptacle the application of at least two polymer composite materials in the monolithic construction of the core of three-layered polymer composite material, which are compatible while still presenting different properties.
C) Introducing sealing layers of resin reinforced with fiber glass as continuous coatings on the inner and outer surfaces of the polymer concrete structural core of the receptacle, with at least 3 structural layers over all inner surfaces and, of course, also reinforced according to industry standards in specific areas or places as joints on overflow boxes or electrolyte feed systems.
In addition, the most important structural stresses to which empty cells are subjected result from point or concentrated overloads of a mechanical nature in their handling, shipping, storage, and installation, or of an accidental nature (drop of electrodes), as well as thermal overloads due to significant sudden and/or localized drops in temperature (thermal shock). The vulnerability of cells to such overloads increases in direct proportion to their length/width ratio.
The design of the improved electrolytic cell receptacles of the invention has been simultaneously optimized both structurally and in regard to corrosion resistance, with absolute impermeability and minimizing heat loss during operation. To achieve these four objectives, computer modeling and analysis according to the finite element method have been used, with temperature data obtained directly from electrolytic processes in industrial operations. Such analysis establishes the essential conditions needed to achieve lightened stresses on the structural material workload with minimal concentrations of stresses during the working life of the receptacle, taking into account all the most severe real operating conditions that are typical in both processes of electrorefining and electrowinning as well as the normal service and handling of both types of empty cells. The optimization of the receptacle is generic and concerns the selection of a combination of such relevant parameters as geometric form, spatial distribution of the volumes of material in such geometric forms, and characteristics and stability of the properties of both the polymer concrete core material and that of the integrated seals that form the three-layered polymer composite material, in such a way as to combine together to significantly increase impermeability, ease of operation, safety, and durability of operation of cells for electrorefining and electrowinning copper and other nonferrous metals at lower cost.
As the only way to achieve improved reliability, ease of operation, and durability of the cells, only those raw materials shall be used that are certified as to their origin, specification, and compatibility, with proven mechanical and chemical suitability for application in cells with corrosive electrolytes; the certification of raw materials and other materials is fundamental to the application of quality assurance standards in all processes and instructions for manufacturing, storing, shipping, and handling.
The ratio of resin/aggregate content in the formulations for polymer concrete materials is reduced, which results in significant improvements in their mechanical properties at the same time as it reduces the cost of the structural core of the receptacle, particularly when we consider that the cost of resin represents at least 70% of the cost of the polymer concrete material.
Resistance to corrosion is significantly improved, and at the same time the absolute impermeability of the receptacles is more than insured over the long term.
Using a three-layered polymer composite material that incorporates monolithic continuous seals on both surfaces, inside and outside the structural core, and mesh reinforcement, all specifying fiberglass of the corrosion resistant (E-CR or similar) class, designed and constructed according to international standards in force in the industry for receptacles of polymer composite materials with high resistance to chemical corrosion.
The formulation of the polymer composite material for the inner chemical barrier seal to insure the absolute impermeability of the receptacle is empirically determined so that the elongation and tensile strength of the multi-layered polymer composite material applied as an inner seal is significantly higher than the adherence of its interface with the polymer concrete material of the structural core, so that any crack that may occur in the polymer concrete structural core is never able to affect the continuity and integrity of the material of the inner seal of the receptacle, thereby insuring absolute impermeability.
Elimination of all inserts, common in the current state of the art, which pass through the seals on the inner surface of the receptacle in contact with electrolytes.
The attachment of the cell to its supports is improved, with a design that ensures restricted movement in both senses in all three directions, without resorting to metal inserts, by incorporating a system based on a xe2x80x9cfusexe2x80x9d component designed to collapse when subject to high stress during significant seismic events, thereby protecting the integrity of the cell.
Depending on which cross-sectional geometry of a conventional cell is used as a referencexe2x80x94for example, the one claimed in (Chilean) Patent No. 38,151xe2x80x94the application of the design of the invention having wide interior and exterior curves to the current horizontal and vertical vertices of the structural core also permits a reduction on the order of 10% in the overall volume of material applied in the new cell receptacle, and accordingly also reduces its weight when compared to the typical reference cell, again lowering costs.
Nevertheless, the overall reduction in the level of stresses (both mechanical and thermal) and the optimal distribution of the volume of the material by using radii at the vertices to prevent the concentration of stresses significantly improve the safety features of the new cell under electrorefining and electrowinning operating conditions.
A basic design concept of the improved electrolytic cell receptacle of the invention is to avoid any concentration or localization of discrete volumes of polymer concrete so as to achieve a clean simple receptacle with uniform thicknesses, moderate transitions, and ample radii in order to thereby manage setting contractions and insure complete and homogeneous curing and easy removal from the mold, and to provide electrolytic cell receptacles for operation that are as relaxed or as free of internal stresses as possible.
In order to improve the distribution of stresses in the polymer concrete core, and above all, in order to be able to reliably repair any possible fissures in the structural core cells produced by catastrophic events, a prewoven mesh is incorporated in the structural core in order to provide bidirectional reinforcement in the plane of the mesh. This prewoven mesh for bidirectional reinforcement is preferably formed of a framework of fiberglass rods of the E-CR class resistant to acid corrosion, pultruded with vinyl ester resin, with a square or hexagonal class section, twisted, or with a circular cross section and surface fibers applied in a spiral braiding, with predetermined spacing and points of contact between the rods of the prewoven mesh adhered using vinyl ester resin. The prewoven mesh is applied before emptying the polymer concrete over the continuous coating seals on the surfaces of the core mold, onto the side and end walls and below the outer surface of the bottom. The spacing of the framework on the bottom plane is denser in order to help ensure the integrity of the bottom material of the cell receptacle during the solidification process of the already consolidated polymer concrete, so as to uniformly distribute contractions and to prevent the formation of cracks caused by setting contractions, which is typical of polymer concrete cells manufactured according to the state of the art.